Method of continuous cellulose digestion and digester adapted for performing the method

ABSTRACT

IN A CONTINUOUS CELLULOSE DIGESTER IMPREGNATION OF CHIPS WITH DIGESTING LIQUOR TAKES PLACE IN A TOP ZONE AND DIGESTION TAKES PLACE IN A LOWER ZONE. A COMPARATIVELY LOW TEMPERATURE IS MAINTAINED OVER AT LEAST THE GREATER PART OF THE IMPREGNATING ZONE BY PREVENTING HEATED LIQUOR FROM THE DIGESTING ZONE TO ENTER THE IMPREGNATING ZONE DUE TO THE CONVECTION, IN A TRANSITION ZONE COOLED LIQUID IS BROUGHT TO FLOW RADIALLY AND HORIZONTALLY OVER THE ENTIRE CROSS-SECTION OF THE CHIPS COLUMN IN ORDER TO SEPARATE THE TWO ZONES IN TEMPERATURE RESPECTS.

May 21, 1974 B. S. E. OSTBERG METHOD OF CONTINUOUS CELLULOSE DIGESTION AND DIGESTER ADAP'I'ED FOR PERFORMING THE METHOD Filed June 29, 1.971

2 Sheets-Sheet l B. 5.5. OSTBERG 3,811,994

METHOD OF CONTINUOUS CELLULOSE DIGESTION AND DIGES'I'ER ADAPTED FOR PERFORMING THE METHOD Filed June 29, 1971 2 Sheets-Sheet 8 United States Patent Oflice 3,811,994 Patented May 21, 1974 Bengt Sven Erik Ostberg, Karlstad, Sweden, assignor to Kamyr Aktiebolag, Karlstad, Sweden Filed June 29, 1971, Ser. No. 157,855 Claims priority, application Sweden, June 29, 1970, 9,004/70 Int. Cl. D21c 3/24 7 Claims ABSTRACT OF THE DISCLOSURE cross-section of the chips column in order to separate the two zones in temperature respects.

The invention relates to a method of continuous cellulose digestion wherein a coherent column of chips or other finely comminuted fiber material mixed with a digesting liquor, such as sulphate or sulphite liquor, during its continuous downward motion through an upright digester is caused to pass first through a zone where the fiber material is impregnated with digesting liquor at a comparatively low temperature and then through a digesting zone where a temperature is maintained which is suitable for the dissolution of the lignin contents of the fiber material.

The object of the invention is to keep the impregnating zone and the digesting zone carefully separated in temperature respects as far as possible and thus to prevent sibly chips emanating from the digesting zone and caused by the tendency of convection of the digesting liquor due to the temperature inversion in the top end of the digester.

According to the invention said object is reached without significant influence upon the impregnation or the digestion by maintaining in a thin layer at the lower .part of the impregnating zone an essentially horizontal flow of liquid over the entire cross-section of the chips column, viz by extracting liquid at the periphery of the chips column, cooling the same in an exterior circulation loop and returning said liquid to the center of the chips column.

The invention also relates to a digester adapted for performing said method, which digester is characterized essentially in that between its upper end provided with a chips feeding device, and a lower zone where the digesting liquor is heated to the full digesting temperature by circulation through an outer loop, there is arranged a liquid circulation loop comprising a strainer inserted in the digester shell, an exterior conduit connected thereto, in which a pump and a cooling means are included, and an interior conduit having an orifice located at the center of the digester cross-section.

The invention will be more closely described herein below with reference to the accompanying drawings, wherein FIG. 1 diagrammatically shows an embodiment of the digester according to the invention, and FIG. 2 shows a modified embodiment of the top of the digester.

The digester shown in FIG. 1 is mainly of standard design, thus consisting of a standing cylindrical container 11 which preferably widens slightly in the downward direction and which is adapted for continuous downward motion of chips or other finely comminuted fiber material mixed with digesting liquor, e.g. sulphate or sulphite liquor, in such a quantity that the mixture moves as a coherent column without turbulence or significant interior eddies or convulsions. The length of the digester is divided into an impregnating zone a, a digesting zone b, and a washing zone 0. At the upper end of the digester there is a charging device comprising a screw feeder 13 and a surrounding cylindrical strainer 15. A fluent mixture of chips and digesting liquor having a temperature of preferably about IOU- C. is supplied through a conduit 17 to the upper end of the feeder screw, and excess of liquid departs through the strainer 13 and the return conduit 19 connected to the space outside said strainer, whereas the thickened chips suspension which may have a liquid to wood ratio of 1.5-3.5 :1, slowly moves on down into the digester without any relative motion of liquor and chips. During said motion through the zone a the chips are impregnated with digesting liquor at the operating pressure of the digester which may be 7 to 15 atmospheres overpressure, preferably about 10 atmospheres overpressure. The length of the impregnating zone and the feeding speed are so chosen that the impregnation is extended over a time period of 10 to 60 minutes, preferably about 25 minutes. During said period the digesting chemicals gradually dilfuse into the chips, so that the concentration of the free liquor drops towards the lower end of the impregnating zone.

When the chips-liquor-mixture leaves zone a and enters digesting zone b, heating thereof takes place to a suitable digesting temperature, to C., depending upon the type of digesting liquor etc. Said heating is caused by digesting liquor being withdrawn through one or the other of the two parallelly connected strainer girdles 21, 23 and is pumped by the pump 28 through a heater 27 wherein the digesting liquor is heated directly by steam or as shown, indirectly by heat exchange with high-pressure steam supplied through the conduit 29, and then on through a conduit 31 suspended centrally in the digester and having its end orifice located at the level of or merely slightly above the strainer girdles 21, 23. At the lower end of the digesting zone b the spent liquor is withdrawn through a strainer girdle 33 and sent through a conduit 35 to a plant for recovery of the chemicals.

In the lowermost zone 0 of the digester, washing of the digested pulp takes place before the same is transferred by means of a discharging device 37 to the pulp outlet conduit 39. The discharge is connected with lowering of the pressure and usually also dilution. The wash is performed by cool wash water supplied close to the pulp outlet as at 39a and forced upwardly through the digester in countercurrent to the pulp. Wash water is withdrawn through a strainer 41 and is pumped through a heating device 43 and an interior tube 45 back to the center of the digester section, where the heated wash liquid is spread radially and distributed over the pulp column. Said heated wash water then performs a second step c f the washing of the pulp, while displacing the spent liquor towards and out through the strainer 33. Immediately below the spent liquor discharge strainer 33 a radial equalization circulation is maintained by withdrawing liquid through a strainer girdle 47 and pumping the same back by means of the pump 49 through the tube 51 to the center of the digester at a level slightly above the strainer 47.

In the above-described cellulose digester, which is previously known in principle, according to the invention measures are taken for performing the impregnation during almost equally favourable conditions as it performed in a separate vessel. Said measures are particularly aiming at maintaining an even temperature over the entire length of the impregnating zone a and also over the cross-section of the digester at the various levels. According to the invention said objects are realized in principle by preventing all kinds of heating of the chips-liquormixture while in zone a. The desired and correct impregnation temperature has been imparted to said mixture already in the conduit 17 and on account of the whirling flow in said conduit the temperature is equalized so that each individual particle has got the right temperature. It is desired to maintain said even temperature distribution during the entire impregnating operation in zone a and it should not be disturbed by any axial flow of liquid of different concentration or temperature. Such disturbances may easily take place e.g. by eddy currents, eruptions or convulsions at the border zone between the zones a and b on account of the updrift or tendency to convection connected with the heavy temperature gradient.

According to the invention said measures consist in maintaining over the entire cross-section of the chips column and in a thin layer closely above the strainers 21, 23 a radial and essentially horizontal flow of centrally supplied liquid having been cooled in an exterior circulation loop. To that end the digester is provided with a strainer girdle 53 for withdrawal of liquid at the periphery of the digester and with a central tube 55 for returning said liquid to the center of the digester cross-section and approximately at the level of the strainer 53 or slightly above the same. By means of the pump 57 the liquid withdrawn by the strainer 53 is caused to flow in an exterior circulation loop comprising a heat exchanger 59 and is then returned to the digester. The heat exchanger 59 functions as a cooler, its secondary side being connected to a conduit 61 carrying cool water or cool liquor. The same quantity of liquid as is withdrawn, is returned in said loop to the digester, i.e. without addition of liquor or dilution and also without draining or evaporation taking place in said loop. The pump 57 is a low-pressure pump that merely has to supply the pressure differential corresponding to the flow resistance in the conduits. A valve 63 inserted in the conduit 64 carrying the cooling liquid is controlled by a temperature sensing instrument 65 so that the temperature of the liquid in the tube 55 is maintained at a desired value. Said temperature is preferably the same or slightly lower than the temperature of the chips-liquor-mixture supplied to the digester top. Ordinarily said temperature is somewhat below 120 C. The cooling circulation should be rather heavy and be chosen in a proper proportion to the quantity of the digesting liquor supplied to the digester per unit of time. Preferably, said quantity of cooled liquid per unit of time is at least equal to said first-mentioned quantity so that the entire quantity of supplied digesting liquor is withdrawn, cooled, and returned at least once, usually several times, before it passes on to the digesting zone. Said heavy cross-flow which is concentrated to a comparatively thin layer will carry with it such digesting liquor of higher temperature as locally and due to convection has penetrated up into said layer, and therefore convection currents cannot enter into zone a and heat said zone locally and uncontrollably. Instead any such convection currents will heat the liquid withdrawn by the strainer 53, but said heat is removed by the cooling taking place in the cooler 59 and will not affect the impregnation. Therefore, the impregnation temperature can be maintained uniform from the digester top to the level where the digesting zone b begins. Of course, a small transition zone will be present between the strainers 53 and 21 but said zone can be held quite small by placing the strainers close together. Of course, the advantages of the invention will be gained also if said transition zone be somewhat extended, but under all circumstances the orifice of the conduit 55 for supply of cooled liquid should be placed above the orifice of the conduit 31 for supply of heated digested liquor and at a distance therefrom, measured in the axial direction, which is considerably less than strainerto the strainer 21 or to the orifice of the conduit 31 for supply of heated digesting liquor. As mentioned, the orifice of the conduit 55 for cooled liquid is situated approximately at the level of the corresponding withdrawal strainer 53. Considering the motion of the chips column it is preferable, however, to place said orifice ,somewhat higher, i.e. upstream of said strainer,. but

usually at a maximum distance therefrom, measured in the axial direction, equal to /3 of the diameterof the digester.

The modified digester top shown in *FIG. 2 corres nds to the embodiment shown in FIG. 1 in respect of charging device, cooling circulation, and heating circulation, and the same reference numerals have been used for corresponding details. The difference consists in that the impregnating zone a has been made longer and narrower in comparison to the standard design of the FIG. 1. In a digester having a diameter D of the digesting zone of 16 feet (4.9 meters), which is a common measure for a digester having a production of 600 tons of pulp per day, the diameter d of the cylindrical top may be about 12 feet (3.65 meters) over a length of about 32 feet (9.8 meters). A-conical shell part 67 joins the narrow top to the lower widened shell part 11 of the digester. The strainer 53 pertaining to the cooling circulation is located at the lower end of the slim top, and the strainer 21 pertaining to the heating circulation is located at the upper end of the wider cylindrical shell part 11 of the digester.

Convection currents of hot liquor which from the digesting zone reach into the transition zone between said two strainers 21, 53 will be diverted and weakened by contact with the conical part of the digester shell and will not so easily find their way to the slim digester top. Moreover, the area of the chips column as well as of the material of the digester shell is relatively reduced which involves less heat transfer by conduction. Finally, the linear speed of the chips column will be higher in the narrow top than elsewhere. Therefore, the transfer of heat from the digesting zone to the impregnating zone will be greatly hampered.

According to FIG. 2, the diameter of the digester top is considerably less than the diameter of the rest of the digester, the reduction being about 25%. However, also a reduction of the diameter as low as 15-20% or still less will be etIective, and of course, a reduction greater than 25% may be chosen, although the top then has to be made inconveniently long for a complete impregnation of the chips therein.

The design of the digester with a long slim top also involves that the capacity of the cooler 59 can be reduced.

What is claimed is:

1. In a method of continuous cellulose digestion, 1n which a coherent column of finely comminuted cellulosic fiber material mixed with digesting liquor during its continuous downward motion through an upright digester is caused to pass first through a zone, wherein the fiber material is impregnated with digesting liquor at a lower temperature compared to a temperature suitable for lignin dissolution, and then through a digesting zone Where a perature is maintained suitable for lignin dissolution, the improvement whereby heated liquor in the digesting zone is prevented from entering the impregnating zone by convection, said improvement consisting in that at the lower end of the impregnating zone there is maintained over the entire cross-section of the fiber material column an essentially radial and horizontal flow of cool liquid by withdrawing liquid at the periphery of said column at the lower end of the impregnating zone dividing said zone from the digesting zone, cooling the withdrawn liquid by heat exchange to a temperature substantially equal to the temperature of the fiber material-liquor-mixture supplied to the digester top, and returning said cooled liquid to the center of said column substantially at the level of the liquid withdrawal whereby the temperature is maintained substantially uniform throughout the impregnating zone.

2. The method as claimed in claim 1, wherein the withdrawn liquid is returned in an unchanged quantity to the center of the fiber material column.

3. The method as claimed in claim 1, wherein the fiber material and the surrounding digesting liquor are moved without relative motion during the passage of at least the greater part of the impregnating zone.

4. In the method as claimed in claim 1, wherein there is maintained over the greater part of the impregnating zone a temperature of IUD-120 C. and the temperature at the transfer to the digesting zone is raised to 150- 180 C., the improvement consisting in that the liquid withdrawn at the lower part of the impregnating zone is cooled to a temperature between 100 and 120 C. before the same is returned to the digester.

5. In the method as claimed in claim 1, wherein the heating of the fiber material to digesting temperature is performed by withdrawal of liquid from the fiber material, heating the same in an exterior circulation loop and returning said liquid to the digester, the improvement consisting in that the supply of circulating cooled liquid takes place at a point situated above the point of supply of heated liquid and at a distance therefrom considerably less or not greater than /3 of the distance to the digester top.

6. The method as claimed in claim 1, wherein the quantity of liquid withdrawn at the periphery of the impregnating zone is at least equal to the quantity of liquid supplied with the fiber material to the digester, so that said liquid on an average is withdrawn, cooled, and returned at least once before it passes on to the digest ing zone.

7. The method as claimed in claim 1, wherein the impregnation is extended over a length of the fiber material column corresponding to its advance during a time period of 10 to minutes.

References Cited UNITED STATES PATENTS 3,448,002 6/ 1969 Bryce et al. 16219 3,578,554 5/1971 Richter 162-49 3,445,328 5/1969 Laakso 16219 ROBERT L. LINDSAY, 111., Primary Examiner P. CHIN, Assistant Examiner US. Cl. X.R. l62237, 248, 250 

